Thermoplastic-photoconductor holographic recording medium has generally been in the form of several transparent layers over a transparent substrate. Thus, a substrate such as NESA glass or a flexible substrate such as Mylar is first coated thereon with an optically transparent electrically conductive layer, then a photoconductive layer, and finally a thermoplastic layer. Previous thermoplastic holographic recording media suffer the problem of decaying charge contrast at the interface of the photoconductive and the thermoplastic layer during hologram development, resulting in low diffraction efficiency or in worse cases no deformation at all.
The charge contrast created by the holographic light pattern through the action of the photoconductor resides at the interface of the thermoplastic layer and the photoconductive layer. During thermal development the charge contrast tends to diminish due to increased electrical conductivity of the thermoplastic layer. As a result, the driving force for the surface deformation is weakened, thereby the deformation is much less than it could be had the charge contrast maintained intact. There exists a large electrical potential difference between the bright fringe and the dark fringe at the interface and this potential difference has a tendency to diminish the charge contrast. At temperatures below the softening temperature of the thermoplastic, however, the electrical conductivity is not high enough to wipe out the charge contrast. For example, with a polyvinyl carbazole (PVK) layer as the photoconductive layer, the electron mobility is sufficiently low at temperatures below 100.degree. C. to prevent charge smearing, but with the thermoplastic layer having a softening temperature of say 50.degree. C., the charge contrast can be wiped out when the softening temperature is reached for a sufficiently long time.
In the copending application Ser. No. 658,324, filed Feb. 17, 1976, now U.S. Pat. No. 4,131,462 in the names of Tzuo-Chang Lee and Jacob W. Lin, entitled "New Device Configuration for Thermoplastic Recording" and assigned to the same assignee as the present invention, the above described limitations are improved upon by the introduction of a new layer between the thermoplastic layer and the photoconductive layer with the following properties: (a) It is non-photoconductive at the wavelength of the holographic recording but is photoconductive at a different wavelength. (b) It is electrically insulating at temperatures used for thermal development, that is, extremely low surface conductivity at the development temperature. The charge contrast created by the holographic exposure will, therefore, reside at the interface of this new layer and the photoconductive layer and during thermal development the charge contrast will remain intact. We can thus have a large driving force for deformation and achieve controllable and appreciable deformation.
After the thermal development, in the copending case, charges will reside across the new layer and they can be removed by using a wavelength to which the new layer is sensitive. An example described therein of fabricating such a device comprised a trinitrofluorenone (TNF) doped PVK layer first coated on, say, indium oxide on glass, and then a special solvent such as xylene was used to leach out an appropriate amount of TNF from the surface of the doped PVK. That procedure generated the new layer mentioned because pure PVK (PVK stripped of TNF) is photoconductive only at ultraviolet (UV) wavelengths and pure PVK is also electrically insulating at temperatures up to 180.degree. C. Advantages of that copending invention are: (a) the hologram diffraction efficiency (.eta.) can be higher, (b) the dynamic range of .eta. vs. heat energy is greatly increased while in prior art .eta. is critically dependent on heat energy, (c) thermoplastic with relatively high electrical conductivity can be used.
A limitation of the procedure shown in the copending application has arisen in that there is a certain lack of repeatability in forming the layer by leaching since the leached layer should be only a few tenths of a um thick. Also, the leached layer actually turns out to be something less than pure PVK throughout the layer as the leaching agent is not equally effective at a depth into the material as on the surface and an increasing gradient of TNF remains proceeding into the leached layer.